In this pilot study investigators propose to explore the effects of a locally delivered inhaled pulmonary vasodilator (inhaled treprostinil) on exercise performance, pulmonary blood flow, venous pressure response and vascular function in stable Fontan patients. Investigators will also assess the effects of resting and acute rises in exercise induced systemic venous pressure on liver stiffness and will also assess whether treprostinil will attenuate the acute stiffness increase that investigators expect to see. This will be a prospective, randomized, double-blinded placebo controlled, crossover trial. Following recruitment and informed consent, each participant will undergo three study visits, including baseline testing (visit 1), followed by two sets of exercise and vascular function tests (visit 2 and visit 3) at CCHMC. Twenty-six patients will be enrolled in this study.